The term “fogging” as used herein is used to describe the evaporation/sublimation and subsequent condensation/deposition of low molecular weight, volatile organic materials from plastic or elastomer components onto cooler adjacent surfaces, and more particularly, where the condensation of said volatile organic materials forms as a film onto the surface of transparent barriers such as windows and light covers, causing a reduction in transparency, thereby impairing the functional effectiveness of the transparent barrier. This phenomenon is well known within the automotive industry, where both plastic and elastomer based products are commonly found within the enclosed and confined spaces of passenger compartments and behind lamp covers. In these examples, fogging is commonly experienced as an unsightly oily film on the interior surface of the vehicle windows or as reduced light transmission from automotive lamps. In the case of automotive headlights, such fogging could have safety implications for drivers and passengers.
Fogging caused by the condensation of low molecular weight, volatile organic materials should not be confused with fogging caused by the condensation of moisture from air onto the interior surfaces of windows, which, because the moisture can again evaporate when ambient conditions allow.
While very little information is published describing common rubber compounding practices to minimize the risk of fogging, a technologist skilled in the art of rubber formulation design will be aware of compounding measures that can be taken to reduce this effect. Such measures include the use of high molecular weight mineral oil, plasticizers having a low aromatic content or the partial/complete replacement of the mineral oil plasticizer with a polymeric plasticizer such as liquid polybutadiene or polyisobutylene. Complete removal of all plasticizers is a further option, but may lead to processing difficulties, or even be prohibitive for certain low hardness applications. Certain carbon blacks containing higher levels of aromatic amines should also be avoided, while the use of plate structured mineral fillers such as talcum can be beneficial by acting as a barrier to the release of volatile substances. In general any additive having a low molecular weight has the potential to cause fogging and should therefore be used at a low level or more preferably avoided. Examples of such ingredients include many process aids, waxes, stabilizers, anti-oxidants, stearates and stearic acid.
Both unreacted (residual) sulphur vulcanization accelerators and peroxides and their decomposition products may also contribute to fogging, necessitating the use of full cure times, preferably followed by a suitable post cure to evaporate any organic volatile materials from the cured product.
WO01/18074 describes a method whereby the use of a combination of peroxides and specific olefinic copolymers with the further addition of one or more anhydride and/or acid groups shows reduced fogging when compared to a control compound.
JP2001151923 describes a method of lowering the decomposition temperature of azodicarbonamide (ADCA) blowing agent in EPDM sponge using a zinc based foaming aid, thereby avoiding the use of a urea foaming aid that is known to be volatile and can therefore cause fogging. Although sulphur cure is preferred, the patent lists other cure system options such as peroxide and alkylphenol-formaldehyde resins. However, although choosing a blowing agent that may give less fogging due to its different decomposition products it still needs high amounts of normal process oils.
The benefit of zeolite as an odor inhibitor and absorber of volatile organic components (VOC) is well documented. Examples can be found in CN 102382363, where zeolite has been added to polypropylene based compositions to reduce the material smell.
JP 2010150417 further describes a propylene polymer composition containing zeolite and having reduced VOC emission, especially formaldehyde and acetaldehyde.
Further use of zeolite in rubber and plastic compositions is made to absorb moisture, VOC's and odors from the environment around the rubber or plastic product, thereby controlling smell and potential fogging emanating from adjacent components.
US 20050014854 describes sulfur and peroxide cured compositions, or thermoplastic rubbers containing crushed zeolite with a particle size of at least 250 mesh, at addition levels of 1 to 2 times the polymer weight, where the compositions are used to produce odor absorbing shoe liners. Further examples of where the use of zeolite is described as an additive to reduce moisture and VOC's is for the application of insulating glass units.
WO97/49779 describes an absorbent containing non-curing composition where depolymerized butyl rubber is used as a carrier for the absorbent materials, added at levels of 20-50% by weight, of which zeolite is listed as moisture absorbent. This non-curing sealant serves to desiccate the space between the glass panes in the window units.
Along a similar theme, U.S. Pat. No. 6,491,992 B1 describes a thermoplastic elastomer vulcanizate composition preferably containing between 10 to 70 parts of synthetic zeolite moisture absorbent for use as a seal and/or a spacer for insulating glass units.
Examples of the use of zeolite as an additive in curable rubber compositions for the purpose of trapping VOC's formed by the decomposition of accelerators for sulphur curing or organic peroxide curatives can also be found. U.S. Pat. No. 7,687,559 B2 describes a rubber composition for the preparation of rubber rollers, which minimizes the content of VOC's formed when the rubber roller is produced by sulphur cross-linking. While organic peroxide is listed as one potential cure system, the Invention described and the examples given focus only on the use of accelerated sulphur cures. Removal of VOC's generated during the curing process is achieved by the Inclusion of at least one component of an acid anhydride and a zeolite. The zeolite used has an average particle size of about 1-50 μm, and is preferably added to the rubber composition at a level of 5-30 parts by weight based on 100 parts of the base rubber.
Patent EP 2 441 798 A1 describes the addition of an activated zeolite to a curable rubber composition comprising a phenol formaldehyde cure system to increase both the rate of cure and the final cross-link density.
Despite the described improvements attributed to the inclusion of zeolite in sulphur and peroxide cured rubber compositions regarding the reduction of emitted VOC's and a lower capacity to cause fogging, development work carried out by the inventors of this application has shown that, while zeolite does indeed give a measurable improvement to gravimetric fogging of sulphur and peroxide cured solid rubber compositions when measured according to the standard described by DIN 75201B, significant fogging still occurs.
Accordingly, there is a need to develop a curable solid rubber composition showing a significantly lower capacity for causing fogging than can be achieved by the use of zeolite in a rubber composition cured with either sulphur or peroxide cure systems.